1. Field of the Invention
The oxide catalyst component preferably consists of 5 to 10 parts by weight of Mn.sub.2 O.sub.3, 10 to 20 parts by weight of CuO, 5 to 10 parts by weight of Fe.sub.2 O.sub.3, 60 to 80 parts by weight of .gamma.-Al.sub.2 O.sub.3 and 1 to 5 parts by weight of MgO. Further at least one of those oxides of Mn.sub.2 O.sub.3, CuO and Fe.sub.2 O.sub.3 can be plated with 1 to 5 percent by weight of Cu, relative to said oxide.
The inventors have found the fact that, at the time of oxidizing carbon monoxide and hydrocarbons contained in exhaust gases arising from internal combustion engines such as automobile engines and various combustion apparatuses, the use of catalyst elements containing some noble metal or metals, for instance, platinum catalyst elements, in combination with catalyst elements not containing any noble metal, for instance, base metal oxide catalyst elements, gives rise to an unexpected effect, namely, decomposition of nitrogen oxide, which is not an inherent property of the individual catalyst elements. The present invention relates to a catalyst accomplished on the basis of this finding. The term `catalyst elements` herein means molded catalysts or unitary objects containing catalyst and carrier, which may take various shapes such as pellet-shape, globe-shape, cylinder-shape, plate-shape, etc.
2. Description of the Prior Art
As is well known, for the purpose of making exhaust gases innocuous, the use of different catalysts for treating the respective components of the exhaust gas, that is, oxidizing catalysts for treating carbon monoxide and hydrocarbons and reducing catalysts for treating nitrogen oxides, has hitherto been popular. Accordingly, in order to make harmless the nitrogen oxides, carbon monoxide and hydrocarbons, inclusive, in exhaust gases there has been employed a two-stage purification method comprising, first, decomposing nitrogen oxides so as to make same harmless by means of reducing catalysts without blowing air therein and, second, oxidizing carbon monoxide and hydrocarbons so as to make them harmless by means of oxidizing catalysts in the presence of secondary air blown therein. However, as a matter of fact, the decomposition ratio of nitrogen oxides even by this method has been usually very low, say ranging from several per cent to 30 or 40 per cent at the utmost, and there are instances in which the oxidizing power of said oxidizing catalysts brings about an increase of the nitrogen oxides content, contrary to what is desired.
Accordingly, it has so far been considered extremely difficult to simultaneously convert carbon monoxide, hydrocarbons and nitrogen oxides, inclusive, to make them harmless, and even the attempt to combine reducing catalysts with oxidizing catalysts in various ways has been abandoned without being given serious consideration. Such being the case, nobody has ever discovered the idea of composing a catalyst bed effective for the decomposition of nitrogen oxides by combining fellow oxidizing catalyst elements which do not show any decomposing effect on nitrogen oxides when used individually.